Apparatus for coating and separating pellets

ABSTRACT

Fully grown nickel pellets are separated from the circulating load in a pellet carbonyl decomposer by discharging the load on to a conical pile of pellets formed between a horizontal circular dam and a circular feed nozzle, a flexible curtain being mounted with its lower edge touching the surface of the pile. The fully grown pellets roll preferentially down the pile and overflow the dam with some undersize pellets, which are separated by screening and returned to circulation.

United States Patent [191 Evans APPARATUS FOR COATING AND SEPARATINGPELLETS [75] Inventor: David Evans, Swansea, Wales [73] Assignee: TheInternational Nickel Company,

Inc., New York, NY.

22 Filed: June 24,1971

21 Appl.No.: 156,288

[52] US. Cl ..l18/48, 23/288 G, 118/303, 209/246, 209/254 [51] Int. Cl..C23c 13/08 [58] Field of Search ..118/48-49.5, 303; 117/1 NQ, 100,106-1072; 23/288 G; 209/254, 246

[56] References Cited UNITED STATES PATENTS 1,037,221 9/1912 Dull..209/254 2,384,932 9/1945 Lechthaler ..23/288 G X [4 1 Mar. 27, 19732,647,044 7/1953 Savage et a1 ..23/288 G 7 2,684,124 7/1954 Hines, Jr..23/288 G X 3,155,542 11/1964 Cordell etal ..ll8/303 FOREIGN PATENTS ORAPPLICATIONS 620,287 3/1949 Great Britain ..1 18/48 PrimaryExaminer-Morris Kaplan Attorney-Maurice L. Pinel ABSTRACT Fully grownnickel pellets are separated from the circulating load in a pelletcarbonyl decomposer by discharging the load on to a conical pile ofpellets formed between a horizontal circular dam and a circular feednozzle, a flexible curtain being mounted with its lower edge touchingthe surface of the pile. The fully grown pellets roll preferentiallydownthe pile and overflow the darn with some undersize pellets, whichare separated by screening and returned to circulation.

1 Claim, 1 Drawing Figure PATENTEDHARZT ms Inventor DAVID E L A/vs 7h zfP M A Hornry APPARATUS FOR COATING AND SEPARATING PELLETS The presentinvention relates to pellet size classification and apparatus therefor,and more particularly to apparatus for classifying carbonyl metallicpellets in a carbonyl decomposer.

In the Mond process for producing nickel through the formation andsubsequent decomposition of nickel carbonyl, as described in BritishPat. No. 620,287, a gas containing nickel carbonyl is brought intocontact in a decomposer with nickel pellets that have been preheated toa temperature above the decomposition temperature of the carbonyl. Thenickel is deposited on the pellets as the pellets pass downwards throughthe reaction chamber of the decomposer in counter-current to a stream ofcarbonyl-containing gas, and the pellets leaving the bottom of thereaction chamber are recirculated through the preheater and then backthrough the reaction chamber.

Deposition of nickel on the pellets causes them to increase in size,and'at any time the circulating charge includes pellets of all sizesranging from minute seed particles to fully grown pellets of therequired product size, say its inch diameter.

In order to separate the pellets that have reached or exceeded apredetermined size, which will be referred to as oversize pellets, thecirculating pellets are allowed to fall on to the top of a conical pileof pellets at the top of the preheater. Oversize pellets tend to rolldown the surface of this cone and thence down a chute and over a screento a product vessel, while the remaining smaller pellets, forming thebulk of the feed, lodge preferentially on the surface of the pile. Whilethe pile is being built up in this way at the top, pellets from thebottom pass downwards into the preheater, so that the height of the pileremains substantially constant and its surface remains in substantiallythe same position. Any undersize pellets that have been carried overwith the oversize pellets pass through the screen and are put back intocirculation.

It has now been discovered that the aforedescribed apparatus forclassifying carbonyl metallicpellets can be improved to provide acleaner separation between undersized and oversized pellets. I

Another object of the invention is to provide improved apparatus forclassifying pellets.

Other objects and advantages will become apparent the vicinity of theapex so that the mixture of pellets can flow over the conical surface.Pellets are removed from the bottom of the reservoir in a uniform mannerso as not to disturb the conical surface and at a rate to maintain thepile at a substantially constant height. The rate of flow of the mixtureof pellets over the surface is controlled so that the oversized pelletsretain sufficient energy to flow over the entire conical surface andover the reservoir periphery while the undersized pellets becomeentrapped in interstices of the conical surface.-

Advantageously, a conical pile of pellets is formed between a horizontalcircular dam and a vertically adjustable circular feed nozzle mountedcentrally above the dam, and a flexible curtain is mounted with itslower edge touching the surface of the cone between the feed nozzle andthe dam. To ensure that the pellets overflow the dam uniformly, it isimportant that the top of the dam is accurately horizontal, andadvantageously it is also adjustable for this purpose. The outside edgeof the dam should preferably be chamfered.

Advantageously the dam forms the upper rim of a cylindrical pelletreservoir from which the pellets pass into the preheater, and thepellets overflowing the darn fall on to one or more helical chutesaround the outside of the reservoir leading to an outlet pipe or pipesto the screen and product vessel.

Referring now to the FIGURE, a cylindrical pellet reservoir 1 is mountedon top of a pellet preheater 2, of which only the top is shown, which inturn is mounted on top of the reaction chamber of a carbonyl decomposer.The reservoir carries around its upper edge a circular dam 3 which canbe adjusted so as to be accurately horizontal. Centrally above the damis mounted a discharge nozzle 4, the height of which can be varied bymeans of three adjusting screws 5 (only one being shown) and a flexiblecircular curtain 6 is mounted on the bottom of a cylindrical holder 7surrounding the nozzle 4. Around the outside of the reservoir 1, betweenthe reservoir and an outer casing 9, are twin chutes in the form ofhelices 8 of opposite hand leading down from the level of the dam to anoutlet pipe 10 leading to a screen (not shown). In order to avoidblockages the inclination of these helices should be at nozzle on to aconical pile of pellets that builds up between the nozzle 4 and the dam3.

The pile builds up at the natural angle of repose of the pellets, and itis important to adjust the height of the nozzle 4 so thatpellets mayflow freely from it on to the pile. If the nozzle-is too low, the pilewill build up around it and the flow of pellets will be impeded and maysoon be stopped. If the nozzle is too high, the pellets fall withexcessive force on to the pile and cause excessive disturbance of thepile, resulting in the larger pellets becoming embedded in the surfaceinstead of rolling freely down the pile, and undersize pellets beingforced past the curtain and rolling on to overflow the dam.

Ideally the nozzle is just clear of .the surface of the pile. Most ofthe pellets leaving the nozzle then begin to roll down the outside ofthepile, but small pellets lodge in the interstices or indentations inthe surface while larger ones roll over the indentations to reach thedarn, where they overflow and fall on to one of the helices and thencepass to the outlet pipe 9.

The nozzle 4 should also be accurately centered above the dam and haveits lower edge horizontal, so

i that the pellets issuing from it are distributed uniformly around thepile.

. Further measures which help to reduce the energy of the pelletsreaching the pile are to :incline the chute :11

allows pellets to roll past and down the surface of thepile butintercepts any pellets that may bounce off the upper part despite allthese precautions. In passing the curtain the pellets are slowed down,thus improving the chances of their lodging in a suitable indentationand thereby making segregation more efficient. The materi al of thecurtain should of course be sufficiently flexible to allow pellets toroll past, and it must resist relatively high temperatures, since thetemperature of the pellets in the pile is about 180C. A suitablematerial is polyester fiber cloth. Advantageously at least the lowerpart of the curtain is divided vertically into strips, and it is alsoadvantageous to construct the curtain in two layers each of which isdivided in this way so that the strips in the two layers overlap.

To monitor the temperature of the pellets a thermocouple 12 is mountedin the top of the casing, which is also provided with a sight-glass 13through which the surface of the pile can be observed. 7

The height of 'the reservoir 1 may if desired be reduced by using two ormore pairs of helical chutes in place of the single pair in the example,with a correspondingly increased number of outlet pipes.

It will be appreciated that in a single passage of the whole charge ofpellets circulating through the decomposer only a proportion of theoversize pellets will be separated, while the remainder becomeincorporated in the pile and are recirculated. Furthermore, although theoversize pellets that preferentially overlow the dam are inevitablyaccompanied by a small number of undersize pellets, these can readily beseparated from the oversize pellets by screening and returning them tocirculation, whereas it would be completely impractical to carry out theseparation by continuously screening the whole circulating load ofpellets.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. In a pellet decomposer for thermally decomposing metal compounds thatare heat decomposable to metal including a preheater for preheatingpellets to a temperature sufficiently high to decompose the metalcompound, a reactor in which the metal compound is thermally decomposedon the preheated pellets and recirculating means for recirculatingcooled pellets from the reactor to the preheater, the improvement whichcomprises pellet classifying means which includes: an opentoppedreservoir having a continuous periphery; a plurality of outlet means inthe bottom of the reservoir for continually removing pellets therefrom;feeding means for feeding a mixture of oversized and undersized pelletsto the center of the reservoir for filling the reservoir and for forminga pellet pile having a conical surface,

apex upwards, so that pellets fed to the pile flow downwardly over theconical surface with oversized pellets flowing over the periphery of thereservoir while the undersized pellets become lodged in interstices ofthe conical surface; and energy-absorbing. means between the peripheryof the reservoir and the feeding means for slowing the flow of pelletsdown the conical surface to increase the chance of lodging all but theoversized pellets in interstices on the pile surface.

